This invention relates generally to the field of near-field scanning optical microscopy (NSOM), and more particularly to the development of and a method of fabricating a micro optical fiber light source which may be used as a light source or alternatively in a variety sensor applications.
Microscopes that employ conventional optical imaging techniques are generally limited in their ability to resolve features that are substantially smaller than approximately 1/2 the wave length of light. Consequently, NSOM techniques have been employed to achieve resolution better than 1/2 the wave length of light. In general, NSOM requires that the aperture diameter be less than the wavelength of light and also that the aperture be positioned in extremely close proximity to the specimen to be examined. NSOM generally relates to the interaction of light impinged upon a sample with the sample to be examined. The strength of the light is generally a function of the probe position relative to the sample.
There are several known methods to designing probes to be used in NSOM. One such method is the use of tapered glass pipettes where an optical aperture is defined at the narrow end of the pipette by metalizing the annular region at the very end of the pipette. A second method is using a single mode optical fiber having a flat end and a highly tapered terminal portion whose outer walls are coated with metal, leaving a bare portion to function as an aperture at the very end of the fiber. The above two design approaches for NSOM probes involve certain inherent limitations. One such limitation is the decrease in signal strength because of the aperture size. A second limitation is that the thickness of the metal layers generally applied to the tapered portion of the probes contribute to the size of the probe tip, thereby enlarging the probe tip.
Several of these disadvantages were addressed in later developments which included the incorporation of fluorescent means into the probe tip. The fluorescent means enabled designers to develop an even smaller aperture than those existing in the above mentioned probes. With respect to optical fibers, there is a limitation on the implantation of the fluorescent means onto the probe tip of an optical fiber. While it is possible to implant fluorescent means onto a tapered optical fiber tip, the implantation methods are limited to one of two means. The first such means is ion implantation of an appropriate metal such as cerium, terbium, and europium. Alternatively, fluorescent means may be implanted onto the optical fiber through use of a borehole in the tip of the optical fiber after which fluorescent means, such as dyes, may be implanted into the bore hole. Neither of these two methods, however, result in a chemical attachment of the fluorescent means to the optical fiber tip.
This invention is directed to an improved micro light source (note than micro as used herein refers to light sources on this order of less than 100 microns) and method of fabrication thereof wherein a process is employed to photo-chemically attach an optically active material to the tip of a significantly tapered optical fiber. This results in an optical fiber light source whose aperture is extremely small yet able to act as an intense light source. Light sources of this type have wide ranging applications across a variety of chemical, biological, and medical fields including use as micro-sensors in NSOM. As a micro-sensor, the light source has excellent detection limits as well as photo stability, reversibility, and millisecond response times. Furthermore, successful applications of light sources as sensors include intracell and intra-embryo measurements. Such micro-sensors also have potential applications which include spatially and temporally resolved chemical analysis and kinetics inside single biological cells and their substructures.
A further aspect of this invention involves a method for manufacturing an optical fiber micro optical fiber light source. This method involves the photo-chemical attachment of an optically active material onto the end surface of an optical fiber cable which has been pulled to form an end with a dimension on the order less than 100 microns. More specifically, photopolymerization has been applied as a means to photo-chemically attach the optically active material. This process allows significant control of the size of the micro light source as the degree of photopolymerization varies in accordance with the reaction time of the polymerization process and the shape of the material photopolymerized onto the optical fiber tip varies in accordance with the intensity of the light coupled to the optical fiber and emitted in the direction of the tip. Furthermore, photo-chemically attaching an optically active material enables the implementation of the micro-light source in a variety of sensor applications. In yet a further aspect of this invention, the photo-chemical attachment process may be performed in multiple stages.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.